1. Field of the Invention
This invention relates generally to the use of seismic sensors in geophysical exploration and more particularly to apparatus for attaching a seismic sensor to a cable.
2. Description of the Related Art
In geophysical (seismic) exploration, a seismic source is used to introduce a seismic signal into the earth. The seismic signal propagates through the earth in the form of a wavefront. As the wave front impinges upon the substrata layers (structural changes), a portion of the wavefront is reflected back to the earth's surface. The reflected waves are recorded by a plurality of spaced-apart sensors. The recorded data is processed to obtain geophysical information about the substrata.
To perform seismic exploration on land, in shallow water and marsh land, geophones are typically used as sensors for recording the reflected waves. Commercially available geophones for use in seismic exploration typically are magnetic type geophones. Such a geophone contains a coil and a magnet, wherein either the magnet or the coil is fixed to a housing, while the other element is freely suspended from the housing. The relative motion between the coil and the magnet produces an electromotive force between the coil and the magnet, which produces an electrical voltage (signal) that is proportional to the velocity of the motion. Such geophones require vertical or near vertical placement for optimal results.
In land seismic exploration, geophones are implanted on the earth's surface so that the geophones will remain vertical and move with the earth in response to seismic disturbances. In shallow waters and marsh lands spikes have been used to implant the geophones on the earth's surface. However, implanting geophones under water is time consuming and in many cases it is not feasible. Gimbal geophones have been designed which are fixedly attached to a cable that carries power and data signals (known in the art as the "bottom cable" or "bay cable") by metal straps and tape. Gimbal geophones ensure proper orientation of the geophone, but attaching such geophones to the cable does not ensure that the geophones will always be in contact with the earth's surface when the cable is placed on the earth's surface under water.
Tests have shown that when a geophone is in contact with the earth's surface, the quality of the signal produced is improved and the signal contains less noise compared to a geophone that is attached to the cable and which geophone is not in contact with the earth's surface.
It is therefore desirable to have an apparatus for mounting a geophone to the bay cable which will ensure that the geophone will be in contact with the earth's surface when such a cable is deployed (placed on the earth's surface), regardless of the bay cable orientation.
The present invention addresses the above-noted problem and provides apparatus for attaching geophones to a cable which ensures that the geophones will remain in contact with the earth's surface regardless of the orientation of the cable itself.